Casting method



R. D. ZINN CASTING METHOD June 9, 1964 2 Sheets-Sheet 1 Filed July 14,1961 Fly. 5

R. D. ZINN CASTING METHOD June 9, 1964 2 Sheets-Sheet 2 Filed July 14,1961 INVENTORZ 2A man/0 0. Z//v/v BY I A T 7' Ol/VE X5 United StatesPatent 015 ice 3,136,831 Patented June 9, 1964 This invention relates toa novel casting technique and more particularly relates to a corestructure whereby castings with complicated coring can be made withcores which are easily removable from the casting after the latter hashardened. The invention is particularly applicable to the casting ofpropellants in solid propellant rocket enginesand will be described interms of casting such propellants. However, it will be apparent from thespecification which follows that the invention is been used wherein thecoreis made of a plurality of keyed segments of a solid material whichcan be removed in pieces. Suchmethods havenot been fully satisfactoryfrom several standpoints. For, one thing, in the removal of such cores,there is always the possibility of damaging the casting since manycastings are made of materials which can be easily damaged. Anotherdisadvantage is that the castings are frequently made of a highlycombustible material and there is always the possibilty of fire orexplosion when heat or mechanical force is applied to the casting toremove the core. Further, many of the prior art methods do not leave acompletely smooth inner surface but the surface frequently has smallridges or other irregularities therein. The core material of many priorart methods contaminates the propellant grain, causing ignitiondifliculties.

In accordance with the present invention, a method and apparatus areprovided for the manufacture of castings wherein the core comprises aflexible member which is made temporarily rigid by filling it with asoluble or meltable filler compound so that the rigidized core is heldto a desired shape during casting but can be collapsed and removedthrough a small opening after the casting has hardened.

A primary object of this invention is to provide a casting methodwherein a core is provided which can be easily removed after the castingis completed.

Another object of the present invention is to provide a casting methodwhich has a high safety factor in core removal.

Still another object of this invention is to provide a casting methodwherein a core is used which leaves a completely smooth surface on thecasting.

Still another object of this invention is to provide an economical corewhich does not utilize expensive materials and which can be fabricatedand used with a minimum of labor.

A still further object of this invention is to provide a core structurewhich does not contaminate the propellant surface.

Another object of this invention is to provide a core structure whichcan be installed and withdrawn through an opening smaller than the core.

Other objects and features of advantage of the present invention will beapparent from the specification which follows.

In the drawings forming part of this application:

FIGURE 1 is a perspective view of a rigidized cor showing the mold inwhich the core is made.

FIGURE 2 is an enlarged sectional view of a core made in accordance withthe present invention.

FIGURE 3 is a sectional view showing the method of making a castingemploying the present invention.

FIGURE 4 is a sectional view similar to FIGURE 3 showing one method ofde-rigidizing the core.

FIGURE 5 is a similar sectional view showing the removal of the flexiblemember from the otherwise finished casting.

FIGURE 6 is a sectional view showing a casting which has beenmanufactured in accordance with the present invention.

FIGURE 7 is a sectional view showing the method of utilizing the core ofthe present invention wherein the core is withdrawn through a smallopening in a motor.

FIGURE 8 is a view similar to FIGURE 7 showing a completed rocket motormade in accordance with the present invention.

Referring now to the drawings by reference characters, there is shown inFIGURE 1 a mold 7 made of segments 8 and 10 which mold is usedfor'making the core of the present invention. In the presentillustration the core is relatively simple and the mold merely has afirst cylindrical section 12 and a second cylindrical section 14connected by a disc-like section 16. It will be understood, of course,that the simple symmetrical shape shown is only for purposes ofillustration and that the mold for the core can be made in substantiallyany desired shape. The mold 7 can be made of any suitable material suchasmetal, wood, plastic, ceramic, plaster or the like. In making thecore, a flexible member 18 is provided which generally conforms to thedesired shape of the finished core or which can be stretched to thedesired shape. The member 18 can be of any flexible material which isinert to the material being cast, which has sufiicient flexibility toconform to the desired shape and suficient mechanical and thermalstability to stand the ultimate casting and/ or curing operation.Preferably the flexible member is a rubber impregnated cloth preformedto the desired shape. The rubber impregnated cloth forms an excellentrelease agent for castable solid propellants. The member 18 is placed inthe core mold 7 and the mold sections are closed around the member 18. Amandrel 20 is then placed through the core and a filler compound 22 isthen run into the flexible member 18, preferably under pressure so thatthe member 18 is made to conform to the inner configuration of the mold.The filler material 22 can be any desired soluble or meltable materialbut is preferably a Water soluble plaster. Although the flexible member18 can be rigidized by filling it in its entirety with the fillercompound in some instances, portions of the rigid filler can be precast.Thus, to make the core shown in FIGURE 2, one might first place acylindrical pre-cast filler element 22a on the mandrel 20, enclose thesame in the mold and then cast in place the center disc-like section22b. A third pre-cast cylindrical section 22c can then be inserted tocomplete the core.

After the core has been completed, the core, now generally designated24, is placed in a mold 26 and the casting compound 28 poured throughthe opening 30. In the instance illustrated, the casting compound 28might well be a rubber-like material with an oxidizer salt therein, asis well known in the solid propellant art. After the mold is filled, itis heated to polymerize the propellant, whereupon it will form the solidpolymerized material 32. At this point, the mandrel 20 is withdrawnleaving a central opening through which Water can be passed as at 34.This dissolves out the filler compound leaving only the flexible member18 in place. Although for purposes of illustration water has been shownas being passed through the center,

an organic solvent might be used in the case of some 1 filler materialsor in the case of other filler materials the filler might be merelyheated and allowed to drip out. If only one end of the core isaccessible, the solvent must be introduced and withdrawn from thes'arne'end; After substantially all of the filler material has beenwithdrawn as' is shown in FIGURE 4, the flexible 'm'ember 18 can now bewithdrawn from the casting as is shown in FIG- URE 5. The mold 26 isthen disassembled leaving the' casting with its irregular opening 38 asis shown in FIGURE 6. In FIGURES 7 and 8 the invention is illustrated inconnection with a rocket motor leavingianopening smaller designated'40is made of fiberglass 42. The core includes a small neck portion 44which is smaller than the desired core. Here the rubberimpregnated-cloth mandrel 46 is inserted through the neck opening 44 andfilled with .a water soluble plaster 48. The propellant 50 is then thanthe desired core. Here a single'piece case generally I poured in placeand cured. The plaster48 is then dissolved out and the cloth 46* canthen be pulled out. This (q) placing in a die a continuousopen-ended-webbing I member capable of being filled to form a solid,openended body, said webbing member having suflicient flexibility toconform to the desired shape of ahollow portion of said die, said diehaving" a passage extending through at least one end thereof to saidhollow portion; a

' (b) inserting a mandrel through said passageiinto said webbingmaterial in said hollow portion whereby 'to envelope the sides of saidmandrel, with said webbing material; 7

'(c) injecting into the space between said' mandrel and the adjacentwebbing a flowable' filler material capable of being treated to form arigid, soluble filler, 'the position of said mandrel beingradjusted suchthat s after said filler is injected the'ends ofjsaid' mandrel remainuncovered;

(d) treating said filler material to cause to harden l I to forms arigid'core; f

('e) molding casting' mateiial about'said core while 7 maintaining theends of said mandrel in an exposed condition and withdrawing saidmandrel to provide a passage entirelythrough said core;

(f)- flowinga-solvent for said filler material thronghr I the PaSOformed wherebyto dissolveandre- 'moveisaidfiller; l, r r I casting soformed;

2. The method of claim wherein the filler material a I Iwater-solubleplasten' i v V 3'. The method of claim 1 wherein thewebbingmember is a rubber impregnated cloth. V

7 References Cited; in the'fil'e of :this patent V UNiTED STATES PATENTS(g) and removing the said fleidble webbing'jfromth'ev

1. A METHOD OF MAKING A CASTING HAVING A HOLLOW PORTION THEREINCOMPRISING: (A) PLACING INA DIE A CONTINUOUS OPEN-ENDED WEBBING MEMBERCAPABLE OF BEING FILLED TO FORM A SOLID, OPENENDED BODY, SAID WEBBINGMEMBER HAVING SUFFICIENT FLEXIBILITY TO CONFORM TO THE DESIRED SHAPE OFA HOLLOW PORTION OF SAID DIE, SAID DIE HAVING A PASSAGE EXTENDINGTHROUGH AT LEAST ONE END THEREOF TO SAID HOLLOW PORTION; (B) INSERTING AMANDREL THROUGH SAID PASSAGE INTO SAID WEBBING MATERIAL IN SAID HOLLOWPORTION WHEREBY TO ENVELOPE THE SIDES OF SAID MANDREL WITH SAID WEBBINGMATERIAL; (C) INJECTING INTO THE SPACE BETWEEN SAID MANDREL AND THEADJACENT WEBBING A FLOWABLE FILLER MATERIAL CAPABLE OF BEING TREATED TOFORM A RIGID, SOLUBLE FILLER, THE POSITION OF SAID MANDREL BEINGADJUSTED SUCH THAT AFTER SAID FILLER IS INJECTED THE ENDE OF SAIDMANDREL REMAIN UNCOVERED; (D) TREATING SAID FILLER MATERIAL TO CAUSE ITTO HARDEN TO FORM A RIGID CORE;